An energy absorbing steering column has a steering column, which has a property to absorb energy. Such an energy absorbing steering column provided in a vehicle is widely known as a means for decreasing shock to a steering wheel. Various structures of the energy absorbing steering columns are widely employed. For example, a ball type, a mesh type, a silicon rubber type, or the like, are utilized. As for the ball type, a ball is provided between an outer tube and an inner tube of a steering column. In the mesh type, a metallic mesh structure is utilized for a steering column. In the silicon rubber type, a silicon rubber is sealed in a lower tube so that the silicon rubber is ejected from a slit when an upper tube moves into the lower tube.
JP3611885B2 (Patent document 1) describes “a shock absorbing steering column, in which a second cylindrical column is fit in a first cylindrical column through a cylindrical spacer and shock energy is absorbed by a relative movement of both columns in an axial direction” as a conventional technique. The Patent document 1 suggests the cylindrical spacer to be as follows. “In the shock absorbing steering column, in which the second cylindrical column is fit in the first cylindrical column through the cylindrical spacer, the spacer is made of synthetic resin material and includes plural protruding portions formed at least one of at an outer circumference and an inner circumference. At least one of an inner circumference of the first column and an outer circumference of the second column contacts the spacer through the protruding portions.” Further, a dimension of the spacer is determined in the Patent document 1.
Further, JPH10(1998)-217980A (Patent document 2) suggests a safety steering column, “in which a material weakening portion is formed so as to be provided at an end portion of a column tube over a circumferential surface, the end portion extending into a passenger compartment, a cone-shaped configuration member is provided adjacent to and coaxially with the end portion of the column tube, a minimum outer diameter of the cone-shaped configuration member corresponds to an inner diameter of the end portion of the column tube and the minimum outer diameter portion of the cone-shaped configuration member faces the end portion of the column tube”.
In the steering column described in the Patent document 1, the spacer made of synthetic resin material is provided between the first cylindrical column and the second cylindrical column. The Patent document describes that, even when the spacer made of synthetic resin material is utilized, a relative inclination of both columns caused by deformation of the protruding portions can be small by determining dimensions of the spacer to certain values. However, a load for starting a movement determined by the spacer made of synthetic resin material is easily affected by change with time and temperature change. In particular, in the Patent document 1, because the spacer made of synthetic resin material is fit-in in a state where the spacer is deformed, deterioration in rigidity with time or heat is of great concern. Further, in the steering column described in the Patent document 1, it is difficult to separately set the load for starting the movement and a load during the movement in the relative movement of both columns in an axial direction.
Further, in the steering column described in the Patent document 2, because a load for starting a movement and a load during the movement of the cone-shaped configuration member are determined from a load of the material weakening portion (for example, a notch) provided over the circumferential surface of the column tube, it is difficult to separately set the load for starting the movement and the load during the movement of the column tube. Further, a dimension in an axial direction necessarily becomes large, which causes low space efficiency and large size, and in turn impracticality. In the meantime, as a cylindrical member for configuring an energy absorbing steering column, various terms, “outer tube” and “inner tube” described above, “first column” and “second column” described in the Patent document 1, or the like, are utilized. Terms “first cylindrical member”, “second cylindrical member”, “outer tube” and “inner tube” will be utilized hereinafter.
A need thus exists for an energy absorbing steering column, in which a load for starting an axial movement of a first cylindrical member relative to a second cylindrical member can be simply and appropriately set. The present invention has been made in view of the above circumstances and provides such an energy absorbing steering column.